SLR - March 2022 - Elias L. Gomatos
Reference: Yu J, Zhao D, Wang S, Zhang C, Huang J, Wang X, Ma X. Shape Approximation and Size Difference of the Upper Part of the Talus: Implication for Implant Design of the Talar Component for Total Ankle Replacement. Biomed Res Int. 2022 Jan 12;2022:1248990. doi: 10.1155/2022/1248990. PMID: 35071589; PMCID: PMC8769841.Level of Evidence: Level III
Scientific Literature Review
Reviewed By: Elias L. Gomatos, DPM
Residency Program: Westside Regional Medical Center – Plantation, FL
Podiatric Relevance: Ankle joint kinematics are largely influenced by the morphology of both the tibial plafond and talar dome. In this study, 3D shape approximation was used as a method to evaluate the talus for total ankle replacement (TAR) implant design purposes. Specific attention was given to the talar trochlea, which was classically described as a “truncated cone” by the work of Inman et al. Several early implant designs have adopted this shape with subsequent instability along the medial and lateral direction and further unknown biomechanical risks. The INBONE implant system added a modification of a “saddle-shaped” truncated cone as referenced by more recent literature. This was further revised with the INBONE II implant system by creating a deeper talar sulcus which provided increased implant stability. The aim of this study was to provide a further understanding of talar morphology consistent with an anatomically functional TAR.
Methods: Data extracted from computed tomography scans of 90 intact ankles was used to approximate the morphology of the talar. 3D model reconstruction was then undertaken for each ankle utilizing Mimics and 3-Matic Medical computer software. Talar shape approximation and curvature analysis was performed using the two-sphere fitting method as standardized in previous studies. A custom Cartesian coordinate system was established within the aforementioned software to further analyze the shape approximation of the medial and lateral articular surfaces of the talus. Finally, accuracy of the 3D models was evaluated using a variety of statistical methods including Euclidean distance comparisons, paired t-tests, Analysis of Variance measures, and a level of statistical significance set to p = 0.05.
Results: Sensitivity analyses revealed that changing the radius of the spheres of the medial articular surfaces of the talus yielded only a minimal impact on the inclination angle. Conversely, when the same analyses were performed to the lateral articular surfaces, larger variations in the inclination angle were seen, suggesting that these parameters should be carefully assessed with implant production. Also, the two-sphere fitting method utilized in this study for 3D reconstruction of the talus presented with the highest maximums in standard deviation.
Conclusions: Since the standard deviation maximums were higher in the two-sphere fitting method, it was found that this presented with the most accurate anatomic similarity to the talar trochlea and shows promise for future implant design bearing this structure in mind. Of note, this study touched upon anthropometric measures which showed that the medial and lateral radii of the articular talus varied somewhat, with a higher incidence of the population possessing a larger lateral sphere radius. The only TAR implant to date with these parameters is the Cadence implant system, which has a lateral talar sphere larger than the medial. Further studies are warranted to test the resilience of implants with these anatomic specifications, but this study in particular calls for the need to factor in the size-specific characteristics of the talus in lieu of simply scaling the talar shape to “custom-fit” a patient based on one generalized model.